2050
Inorg. Chem. 1999, 38, 2050-2056
Thermodynamic Calculations for Reactions Involving Hydrogen Halide Polymers, Ions, and Lewis Acid Adducts. 3. Systems Constituted from Al3+, H+, and ClW. David Chandler* and Keith E. Johnson Department of Chemistry, University of Regina, Regina, Saskatchewan S4S 0A2, Canada ReceiVed June 10, 1998
Semiempirical (MNDO, AM1 and PM3), ab initio (HF/3-21+G(d,p), MP2/3-21+G(d,p), HF/6-311+G(d,p), and MP2/6-311+G(d,p)), and density functional (BVWN/3-21+G(d,p) and BVWN/6-311+G(d,p)) energy minimization and frequency calculations have been carried out on AlCl3, AlCl4-, AlCl2+, HAlCl4, HAlCl3+, and ClHAlCl4-. The same computational set, except for MP2/6-311+G(d,p) frequency calculations, was performed on Al2Cl6 and Al2Cl7-. For ClHAl2Cl7-, only semiempirical calculations, as well as Hartree-Fock and BVWN optimizations using the 3-21+G(d,p) and 6-311+G(d,p) basis sets, were carried out along with Hartree-Fock frequency determinations. ∆G°, ∆H°, and ∆S° values have been calculated for a number of reactions involving these species along with a variety of ions derived from HCl. Thermodynamic data and equilibrium constants at 298 K are presented for 22 reactions using calculations at the best level, along with a comparison of ∆G° values obtained from all molecular orbital methods used.
Introduction Mixtures of AlCl3 and ZCl, where ZCl is an alkali chloride or an organic chloride such as 1-ethyl-1H-imidazolium chloride (ImCl), are Lewis acidic when AlCl3 is in excess and Lewis basic when ZCl is in excess.1 The governing equilibria are
Cl- + AlCl3 f AlCl4-
(1)
AlCl3 + AlCl4- f Al2Cl7-
(2)
and the combination reaction
Cl- + Al2Cl7- f 2AlCl4-
(3)
with K3 ) [AlCl4-]2/[Cl-][Al2Cl7-]. While insignificant at lower (1) Hussey, C. L. In AdVances in Molten Salt Chemistry; Mamantov, G., Ed.; Elsevier: Amsterdam, 1983; Vol. 5, pp 185-229. (2) Boxall, L. G.; Jones, H. L.; Osteryoung, R. A. J. Electrochem. Soc. 1973, 120, 223. (3) (a) Karpinski, Z. J.; Osteryoung, R. A. Inorg. Chem. 1984, 23, 1491. (b) Hussey, C. L.; Scheffler, T. B.; Wilkes, J. S.; Fannin, A. A., Jr. J. Electrochem. Soc. 1986, 133, 1389. (c) Melton, T. J.; Joyce, J.; Maloy, J. T.; Boon, J. A.; Wilkes, J. S. J. Electrochem. Soc. 1990, 137, 3865. (d) Chang, D. R.; Elias, A. M.; Carlin, R. T., Wilkes, J. S. In Proceedings of the Ninth International Symposium on Molten Salts, San Francisco, CA, May 1994; Hussey, C. L., Newman, D. S., Mamantov, G., Ito, Y., Eds.; The Electrochemical Society Inc.: Pennington, NJ, 1994; pp 132-137. (4) Campbell, J. L. E. Ph.D. Thesis, University of Regina, Regina, SK, 1994. (5) (a) Smith, G. P.; Dworkin, A. S.; Pagni, R. M.; Zingg, S. P. J. Am. Chem. Soc. 1989, 111, 525. (b) Smith, G. P.; Dworkin, A. S.; Pagni, R. M.; Zingg, S. P. J. Am. Chem. Soc. 1989, 111, 5075. (6) (a) King, D.; Mantz, R.; Osteryoung, R. J. Am. Chem. Soc. 1996, 118, 11933. (b) Koronaios, P.; King, D.; Osteryoung, R. A. Inorg. Chem. 1998, 37, 2028. (7) Shuppert, J. W.; Angell, C. A. J. Chem. Phys. 1977, 67, 3050. (8) (a) Goledzinowski, M.; Birss, V. I. Ind. Eng. Chem. Res. 1993, 32, 1795. (b) Elrutb, M.; Johnson, K. E. Manuscript in preparation. (9) Campbell, J. L. E.; Johnson, K. E. J. Am. Chem. Soc. 1995, 117, 7791. (10) Campbell, J. L. E.; Johnson, K. E. Inorg. Chem. 1993, 32, 3809.
temperatures, Al3Cl10- becomes more of a contributor at higher temperatures in very acidic melts.3,4,24d For ZCl ) NaCl, the mixtures are liquid over a fair composition range at 225 °C with K3 ≈ 107,2 while for ZCl ) ImCl, mixtures are liquid for XAlCl3 between 0.33 and 0.67 at ambient temperatures with K3 ≈ 1016-1018.3 The addition of HCl to Lewis basic AlCl3-ImCl mixtures has been shown to produce Brønsted acidic (but still Lewis (11) Campbell, J. L. E.; Johnson, K. E.; Torkelson, J. R. Inorg. Chem. 1994, 33, 3340. (12) Emsley, J. Chem. Soc. ReV. 1980, 9, 91. (13) Tuck, D. Prog. Inorg. Chem. 1968, 9, 161. (14) Chandler, W. D.; Johnson, K. E.; Fahlman, B. D.; Campbell, J. L. E. Inorg. Chem. 1997, 36, 776. (15) Chandler, W. D.; Johnson, K. E.; Campbell, J. L. E. Inorg. Chem. 1995, 34, 4943. (16) (a) Fahlman, B. D. B. Sc. (Hon). Thesis, University of Regina, Regina, SK, 1996. (b) Johnson, K. E.; Chandler, W. D.; Fahlman, B. D. In Proceedings of the Tenth International Symposium on Molten Salts, Los Angeles, CA, May 1996; Carlin, R. T., Deki, S., Matsunaga, M., Newman, D. S., Selman, J. R., Stafford, G. R., Eds.; The Electrochemical Society Inc.: Pennington, NJ, 1996; pp 92-103. (17) Rankin, K. N. B. Sc. (Hon). Thesis, University of Regina, Regina, SK, 1997. (18) (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Wong, M. W.; Foresman, J. B.; Robb, M. A.; HeadGordon, M.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92/ DFT, Revision G.4; Gaussian Inc.: Pittsburgh, PA, 1993. (b) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revisions B.1 and D.3; Gaussian Inc.: Pittsburgh, PA, 1994. (19) Pople, J. A.; Schlegel, H. B.; Krishnan, R.; DeFrees, D. J.; Binkley, J. S.; Frisch, M. J.; Whiteside, R. A.; Hout, R. F.; Hehre, W. J. Int. J. Quantum Chem. Symp. 1981, 15, 269. (20) Heidrich, D.; van Eikema Hommes, N. J. R.; von Rague´ Schleyer, P. J. Comput. Chem. 1993, 14, 1149. (21) Atkins, P. W. Physical Chemistry, 6th ed.; Freeman: New York, 1998.
10.1021/ic980640r CCC: $18.00 © 1999 American Chemical Society Published on Web 04/15/1999
Systems Constituted from Al3+, H+, and Clbasic) systems with Hammett acidities close to the range of concentrated hydrochloric acid,4 while the addition of HCl to Lewis acidic mixtures yields superacidic systems of strength comparable to liquid HF.5 The acidity of HCl in various neutral buffered melts has also been studied.6 Mixtures of AlCl3 and pyridinium chloride are Brønsted acidic without HCl addition and appear to be even stronger superacids when AlCl3 is in excess, although quantitative data are lacking at present.7,8 We have shown by HCl solubility measurements,9 1H NMR spectroscopy,10 IR spectroscopy,11 and electrochemistry9 that the protic species present in AlCl3-ImCl are HCl2-, H2Cl3-, H3Cl4-, and ClHAlCl4- in Lewis basic systems, ClHAlCl4- in Lewis neutral systems, and ClHAl2Cl7- and ClHAlCl4- in Lewis acidic systems. Our evidence points to ClHAl2Cl7- as the superacid. Although there is ample evidence for the formation of species HnCln+1- in the literature, it is quite spotty.12 There are precedents for the formation of species XHY- 13 but not with Y- ) bulky chloroaluminate ion. To support our experimental observations of these systems, we embarked on a program of calculating the geometries, energies, and vibrational frequencies of the various ions and molecules believed to be involved. Elsewhere we have reported the data for (HCl)n, (HnCln+1)-, and (Hn+1Cln)+ 14 and the analogous fluorine species,15 for which more experimental comparisons were available. Theoretical data have also been obtained for ZCl-BCl3-HCl and ZF-BF3-HF systems, but the experimental scope here is limited.16 In this paper we present ab initio, density functional, and semiempirical SCF calculations for AlCl3, Al2Cl6, AlCl4-, Al2Cl7-, AlCl2+, HAlCl4, HAlCl3+, HAlCl5- (which will be written as ClHAlCl4-), and HAl2Cl8- (ClHAl2Cl7-). In particular, we discuss the significance of these species to ambienttemperature melt studies in the light of the ∆G° and K values for key reactions. Energy differences for some of these reactions (often without thermal and zero-point energy corrections) have appeared in the literature (vide infra). However, as we have noted previously, PV work and T∆S terms can be significantly important so that ∆G is a much more meaningful parameter in assessing equilibria than the often-reported ∆E or ∆H.14,15 The (22) (a) Ystenes, M.; Rytter, E.; Menzel, F.; Brockner, W. Spectrochim. Acta 1994, 50A, 233. (b) Williams, S. D.; Harper, W.; Mamantov, G.; Tortorelli, L. J.; Shankle, G. J. Comput. Chem. 1996, 17, 1696. (c) Hassanzadeh, P.; Citra, A.; Andrews, L.; Neurock, M. J. Phys. Chem. 1996, 100, 7317. (d) Otto, A. H.; Steiger, T.; Schrader, S. Chem. Commun. 1998, 391. (e) Ystenes, B. K.; Ystenes, M.; Scholz, G. Spectrochim. Acta 1995, 51A, 2481. (f) Shalabi, A. S.; Kamel, K. A.; Assem, M. M. Theor. Chim. Acta 1995, 91, 73. (g) Petrie, S. J. Phys. Chem. A 1998, 102, 7828. (23) (a) Alvarenga, A. D.; Soboungi, M.-L.; Curtiss, L. A.; Grimsditch, M.; McNeil, L. E. Mol. Phys. 1994, 81, 409. (b) Ystenes, M.; Westberg, N.; Ehrhardt, B. K. Spectrochim. Acta 1995, 51A, 1017. (c) Akdeniz, Z.; Pastore, G.; Tosi, M. P. Phys. Chem. Liq. 1996, 32, 191. (24) (a) Blander, M.; Bierwagen, E.; Calkins, K. G.; Curtiss, L. A.; Price, D. L.; Saboungi, M.-L. J. Chem. Phys. 1992, 97, 2733 and references therein. (b) Takahashi, S.; Curtiss, L. A.; Gosztola, D.; Koura, N.; Loong, C.-K.; Saboungi, M.-L. In Proceedings of the International Symposium on Molten Salt Chemistry and Technology; The Electrochemistry Society Inc.: Pennington, NJ, 1993; pp 622-631. (c) Bock, C. W.; Trachtman, M.; Mains, G. J. J. Phys. Chem. 1994, 98, 478. (d) Takahashi, L. A.; Curtiss, L. A.; Gosztola, D.; Koura, N.; Saboungi, M.-L. Inorg. Chem. 1995, 34, 2900. (e) Carper, W. R.; Mains, G. J.; Piersma, B. J.; Mansfield S. L.; Larive, C. K. J. Phys. Chem. 1996, 100, 4724. (25) Takahashi, S.; Saboungi, M.-L.; Suzuya, K.; Koura, N. In Proceedings of the Ninth International Symposium on Molten Salts, San Francisco, CA, May 1994; Hussey, C. L., Newman, D. S., Mamantov, G., Ito, Y., Eds.; The Electrochemical Society Inc.: Pennington, NJ, 1994; pp 69-76.
Inorganic Chemistry, Vol. 38, No. 9, 1999 2051
Figure 1. Structures of AlCl3, AlCl4-, AlCl2+, and HAlCl3+ determined by the MP2/6-311+G(d,p) ab initio method. Bond lengths are in angstroms and angles in degrees.
Figure 2. Structures of Al2Cl7- and Al2Cl6 determined by the MP2/ 6-311+G(d,p) ab initio method. Bond lengths are in angstroms and angles in degrees.
extension of these calculations to liquid HCl, molten NaAlCl4, and molten LiCl-KCl temperatures is under way.17 Calculations Ab initio, density functional, and semiempirical calculations were performed using GAUSSIAN 92/DFT and GAUSSIAN 94.18 Geometries of all species, except for ClHAl2Cl7-, were optimized using MNDO, AM1, and PM3 semiempirical methods, as well as with 3-21+G(d,p) and 6-311+G(d,p) basis sets at the Hartree-Fock and MP2 ab initio levels and BVWN or Becke3LYP density functional procedures. Optimizations for ClHAl2Cl7- were performed using all methods except MP2/6-311+G(d,p). Vibrational frequencies were also obtained using the same methods and basis sets except (a) MP2/6311+G(d,p) frequency calculations were not possible for Al2Cl6 and Al2Cl7-; (b) no post-Hartree-Fock or density functional frequency computations were feasible for ClHAl2Cl7-. From these latter calculations, it was possible (a) to determine that a structure represented a true minimum and not a saddle point in the energy surface; (b) to obtain
2052 Inorganic Chemistry, Vol. 38, No. 9, 1999
Chandler and Johnson
Figure 3. Structure and conformation of HAlCl4 determined by the MP2/6-311+G(d,p) ab initio method. Bond lengths are in angstroms and angles in degrees.
Figure 5. Structure of ClHAl2Cl7- determined by the MP2/3-21+G(d,p) ab initio method. Bond lengths are in angstroms and angles in degrees.
Figure 4. Structure and conformation of HAlCl5- (ClHAlCl4-) determined by the MP2/6-311+G(d,p) ab initio method. Bond lengths are in angstroms and angles in degrees. entropies (S°) as well as zero-point energies and thermal corrections at 298 K and 1 atm pressure. All ab initio and density functional energies were corrected for thermal energies, including zero-point vibrations (Hartree-Fock zero-point energy values were scaled by 0.9019 and MP2 values by 0.9520), to give U values at 298 K. Reaction internal energies (∆U°) were converted to reaction enthalpies (∆H°) by means of eq 4, which assumes ideal-gas behavior for all species.21 In Gaussian 94 the conversion of U to H values, with thermal corrections, is carried out automatically. ∆S° values were obtained using S°s taken from frequency calculations for polyatomic ions and molecules and employing the Sackur-Tetrode equation for monatomic ions.21
∆H° ) ∆U° + ∆nRT
(4)
Results Figures 1-6 provide the structures and conformations of the species studied as determined from MP2/6-311+G(d,p) optimizations, except for ClHAl2Cl7-, where MP2/3-21+G(d,p) was the method used. Structures and energies for AlCl3 (C3V),22 Al2Cl6 (D2d),22a-b23 AlCl4- (Td),22a,b,d,23c,24 Al2Cl7(C2),22b,23c,24a-b,d,e,25 AlCl2+ (D∞h),22g,23c,26 and HAlCl4 (Cs)22d (26) Petrie, S. J. Chem. Phys. 1997, 107, 3042.
Figure 6. Conformation of ClHAl2Cl7- determined by the MP2/321+G(d,p) ab initio method. Dihedral angles are in degrees.
have been reported recently from various calculations, and our results are essentially the same as the most reliable of these. The agreement with experimental data, where available, is exceptionally good in most cases (Table 1). The structures of ClHAlCl4- and ClHAl2Cl7-, which have not been reported previously, are essentially those of the anions, AlCl4- and Al2Cl7-, to which HCl has been attached through a “long” H-Cl bond of 2.3 and 2.4 Å, respectively (Figures 4-6). For this reason, the formulas ClHAlCl4- and ClHAl2Cl7- have been used to represent these anions. The preferred conformation of ClHAlCl4- is “staggered”, belonging to the Cs point group; the dihedral angle involving the H, the hydrogen-bonded Cl, Al, and one of the other Cl’s is 180°. The Cl-H-Cl-Al dihedral angle is also 180°. Although the lowest energy conformation, it is only of the order of 1 kJ/mol more stable than the “eclipsed” form. The energy surface of the ClHAl2Cl7- anion is more complex than any of the other species studied, and several structures appear as local minina with energies differing by only 1 kJ/
Systems Constituted from Al3+, H+, and Cl-
Inorganic Chemistry, Vol. 38, No. 9, 1999 2053
Table 1. Comparison of Experimental and Calculated Structures structural parametersa species AlCl3 (C3V) AlCl4- (Td)
parameter
exp
Al-Cl Al-Cl
2.068 2.12 2.127-2.132 2.09-2.16 + AlCl2 (D∞h) Al-Cl 2.05-2.25 Al2Cl6 (D2d) Al-Clt 2.065 Al-Clb 2.252 Al-Al 3.102 ∠AlClbAl 89.0 ∠ClbAlClt 109.7 ∠CltAlClt 123.4 Al2Cl7- (C2) Al-Clt 2.97-2.13 Al-Clb 2.22-2.29 ∠AlClbAl 110.8-115.1
